Glucose-dependent insulinotropic polypeptide (GIP)
GIP, gastric inhibitory polypeptide (historical), GIP (1-42), incretin
GIP is a 42-amino-acid hormone released from K-cells in the duodenum and upper jejunum in response to nutrients, especially fat and glucose. Together with GLP-1 it accounts for the incretin effect, the observation that oral glucose provokes far more insulin than the same amount given intravenously. Its defining clinical fact is that its insulin-releasing action is largely lost in type 2 diabetes, which redirected drug development toward GLP-1 for two decades before GIP receptor pharmacology returned to prominence.
Mechanism
GIP is cleaved from proGIP in enteroendocrine K-cells, which are concentrated in the duodenum and proximal jejunum, anatomically upstream of the L-cells that make GLP-1. Nutrient absorption, particularly of long-chain fatty acids and glucose, triggers release within minutes of eating. GIP acts at the GIP receptor, a class B G-protein-coupled receptor coupled principally to Gs. On pancreatic beta cells, receptor activation raises cyclic AMP and amplifies glucose-stimulated insulin secretion. Critically, this amplification is glucose-dependent, so GIP alone does not cause hypoglycaemia. Like GLP-1, GIP is a substrate for dipeptidyl peptidase-4, which cleaves the N-terminal Tyr-Ala dipeptide to yield the inactive GIP(3-42).
What distinguishes GIP from GLP-1 is the breadth of its extrapancreatic actions. GIP receptors are abundant on adipocytes, where GIP promotes lipoprotein lipase activity, fatty acid uptake and triglyceride storage, and enhances the postprandial blood-flow response in adipose tissue. GIP is, in effect, a nutrient-partitioning signal that buffers dietary lipid. GIP receptors are also present on bone, where GIP inhibits osteoclastic resorption, and in the central nervous system, including the hypothalamus and area postrema, where GIP receptor signalling influences food intake and nausea pathways. Unlike GLP-1, GIP is glucagonotropic at low or normal glucose concentrations, contributing to counter-regulation, while remaining insulinotropic when glucose is high. This bidirectional, glucose-dependent regulation of both islet hormones is a genuinely distinctive feature.
What the research shows
GIP was isolated from porcine intestine in the early 1970s and named gastric inhibitory polypeptide for its ability to inhibit gastric acid secretion, an effect only seen at supraphysiological doses. It was renamed glucose-dependent insulinotropic polypeptide after Dupre and colleagues demonstrated in 1973 that it stimulates insulin secretion in humans, preserving the acronym while correcting the biology.
The single most consequential finding came from Nauck and colleagues in 1993. Comparing GIP and GLP-1 infusions in nine patients with type 2 diabetes and nine matched controls, they found the maximum insulinotropic effect of GIP was 54 per cent lower in the diabetic patients, whereas GLP-1 retained around 71 per cent of the normal response, a non-significant difference. This asymmetry (GIP resistance with relative GLP-1 preservation) is why the entire incretin drug industry was built on GLP-1 and why GIP was written off as a target for two decades. Later work by Højberg and colleagues showed the GIP defect is at least partly reversible: four weeks of near-normalisation of blood glucose improved the insulin response to GIP, indicating the resistance is a consequence of glucotoxicity rather than a fixed lesion. Christensen and colleagues then characterised GIP as bifunctional in humans: glucagonotropic at low and normal glucose, insulinotropic at high glucose.
The modern situation is more interesting and genuinely unresolved. Both GIP receptor agonism and GIP receptor antagonism improve body weight and metabolic outcomes in preclinical models, an apparent paradox that has generated a substantial literature and several competing explanations, including receptor desensitisation by sustained agonism, differing contributions of central versus adipose GIP receptors, and the possibility that both interventions converge on reduced GIP signalling in adipose tissue. Clinically, dual GIP/GLP-1 receptor agonism has produced weight and glycaemic outcomes exceeding GLP-1 receptor agonism alone in phase 3 trials, while GIP receptor antagonist antibodies have also progressed into clinical development. Native GIP itself remains a physiological probe, not a therapy, and no one has proposed developing it as one.
Evidence assessment
Limited evidence
Native GIP has never been developed as a therapeutic and there are no efficacy trials of it. The human evidence consists of short mechanistic infusion and clamp studies in single-digit to low-double-digit numbers of participants: excellent physiology, but not therapeutic evidence. This tier reflects the therapeutic evidence for the native peptide specifically. It should not be read as a comment on GIP receptor pharmacology in general, where engineered dual GIP/GLP-1 receptor agonists have very strong phase 3 evidence; that evidence belongs to those molecules, not to native GIP, and no such trial is cited in this record.
Tiers are applied consistently across the library and re-checked when new trials read out. Read the grading method.
Key studies
Preserved incretin activity of glucagon-like peptide 1 [7-36 amide] but not of synthetic human gastric inhibitory polypeptide in patients with type-2 diabetes mellitus Preclinical only
The maximum insulinotropic effect of GIP was 54 per cent lower in type 2 diabetes (p<0.05), while GLP-1 retained approximately 71 per cent of the normal response, a non-significant difference. The finding that redirected two decades of incretin drug development away from GIP.
Stimulation of insulin secretion by gastric inhibitory polypeptide in man Preclinical only
Demonstrated that GIP stimulates insulin secretion in humans, establishing it as an incretin and prompting the renaming from gastric inhibitory polypeptide to glucose-dependent insulinotropic polypeptide.
Four weeks of near-normalisation of blood glucose improves the insulin response to glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide in patients with type 2 diabetes Preclinical only
The blunted insulin response to GIP improved substantially after four weeks of near-normal glycaemia, showing that GIP resistance in type 2 diabetes is at least partly a reversible consequence of glucotoxicity rather than a fixed defect.
Glucose-dependent insulinotropic polypeptide: a bifunctional glucose-dependent regulator of glucagon and insulin secretion in humans Preclinical only
GIP is glucagonotropic at low and normal glucose and insulinotropic at high glucose, a bidirectional, glucose-dependent action that distinguishes it sharply from GLP-1 and is central to interpreting GIP receptor pharmacology.
Safety
Short intravenous infusions of native GIP in research settings are generally well tolerated, with no consistent pattern of adverse effects reported at physiological or modestly supraphysiological doses. Because GIP amplifies insulin secretion only when glucose is elevated, it does not cause hypoglycaemia on its own; however, it raises glucagon at low and normal glucose, which is a physiological counter-regulatory effect rather than an adverse one. The theoretical concerns that attach to GIP are metabolic rather than acute: chronic GIP receptor stimulation promotes triglyceride storage in adipose tissue and has been implicated in diet-induced obesity in animal models, which is part of why the agonist-versus-antagonist question remains live. There is no long-term human safety dataset for native GIP because no one has administered it chronically. GIP sold as a research chemical has no licensed status, no evidence base for any human use, and a half-life of a few minutes that makes any consumer application pointless. Anyone interested in GIP receptor pharmacology for metabolic reasons should understand that the licensed medicines in this space are engineered dual agonists with long half-lives and their own extensive trial evidence. They are not native GIP and cannot be substituted by it.
Regulatory status
| Jurisdiction | Status |
|---|---|
| United Kingdom | Native GIP has no UK marketing authorisation and is not a licensed medicine. Material sold to consumers is an unlicensed research chemical. Its 5-to-7-minute half-life makes intermittent administration pharmacologically futile, quite apart from the absence of any evidence of benefit. |
| United States | Native GIP is not an approved medicine and has no FDA-approved indication. It is available as a research reagent only. Engineered dual GIP/GLP-1 receptor agonists are separately approved, but they are distinct molecules with their own evidence and labelling. |
| WADA (sport) | Not listed on the WADA Prohibited List. GIP receptor agonists are not covered by any current category and there is no established performance-enhancing rationale. |
Questions
It was originally called gastric inhibitory polypeptide because it inhibited gastric acid secretion, but that effect only appears at doses far above physiological concentrations. Once Dupre and colleagues showed in 1973 that it stimulates insulin release in humans, it was renamed glucose-dependent insulinotropic polypeptide, conveniently keeping the same acronym while correcting the biology.
Nauck's 1993 study showed the insulinotropic effect of GIP is reduced by 54 per cent in type 2 diabetes while GLP-1's is largely preserved. Later work by Højberg and colleagues showed the defect is at least partly reversible with four weeks of near-normal glycaemia, so it appears to be a consequence of glucotoxicity and receptor downregulation rather than a fixed lesion.
This is genuinely unresolved and one of the more interesting open questions in metabolic pharmacology. Both agonism and antagonism improve outcomes in animal models. Dual GIP/GLP-1 receptor agonists have strong clinical results, while GIP receptor antagonist antibodies have also entered clinical development. Proposed reconciliations include receptor desensitisation by chronic agonism and opposing roles for central versus adipose GIP receptors.
No, and the distinction matters. Those medicines are engineered peptides designed to activate both GIP and GLP-1 receptors with half-lives measured in days. Native GIP is a 42-residue hormone with a half-life of about five minutes and no therapeutic evidence base at all. The strong trial evidence belongs to the engineered molecules, not to native GIP.